It is particularly preferred to employ Staphylococcal genes and gene products as targets for the development of antibiotics. The Staphylococci make up a medically important genera of microbes. They are known to produce two types of disease, invasive and toxigenic. Invasive infections are characterized generally by abscess formation effecting both skin surfaces and deep tissues. S. aureus is the second leading cause of bacteremia in cancer patients. Osteomyelitis, septic arthritis, septic thrombophlebitis and acute bacterial endocarditis are also relatively common. There are at least three clinical conditions resulting from the toxigenic properties of Staphylococci. The manifestation of these diseases result from the actions of exotoxins as opposed to tissue invasion and bacteremia. These conditions include: Staphylococcal food poisoning, scalded skin syndrome and toxic shock syndrome.
The tRNA synthetases have a primary role in protein synthesis according to the following scheme:    Enzyme+ATP+AA⇄Enzyme.AA-AMP+PPi    Enzyme.AA-AMP+t-RNA⇄Enzyme+AMP+AA-t-RNA in which AA is an amino acid.
Inhibition of this process leads to a reduction in the levels of charged tRNA and this triggers a cascade of responses known as the stringent response, the result of which is the induction of a state of dormancy in the organism. As such selective inhibitors of bacterial tRNA synthetase have potential as antibacterial agents. One example of such is mupirocin which is a selective inhibitor of isoleucyl tRNA synthetase. Isolation of tRNA synthetase allows for the identification and analysis of potential antibacterial targets to facilitate screening for antibacterial compounds.
Isoleucyl tRNA synthetase, isolated from Staphylococcus aureus, has already been described (Chalker, A., F., Ward, J., M., Fosberry, A., P. and Hodgson, J., E. 1994 Gene 141:103-108).
Clearly, there is a need for factors that may be used to screen compounds for antibiotic activity and which factors may also be used to determine their roles in pathogenesis of infection, dysfunction and disease. There is a need, therefore, for identification and characterization of such factors and their antagonists and agonists which can play a role in preventing, ameliorating or correcting infections, dysfunctions or diseases.
The polypeptides of the invention have amino acid sequence homology to a known methionyl tRNA syntetase protein from Bacillus subtilis. 